DESCRIPTION OF THE PRIOR ART
Historical developments of Surface Forces Apparatuses (SFA's)
Surface forces apparatuses (SFA's) are instruments that measure the forces between surfaces. They have been in existence since the mid-1950's. Until the 1970's, SFA's measured the forces between two surfaces in air or in vacuum. Gradual improvements in surface sample preparation (e.g., producing smoother surfaces) and in distance-measuring techniques resulted in a series of improved SFA's, and today these can measure the normal force between two macroscopic surfaces (force accuracy: approx. 10.sup.-8 N) as a function of surface separation (distance accuracy: approx. 1 .ANG.). Forces are generally obtained by measuring the deflection of a cantilever spring using an optical or electrical position-sensitive measuring technique, and the separation between the two interacting surfaces, as well as their shape, is measured with a standard optical technique that uses multiple beam interference fringes (known as "FECO".ident."Fringes of Equal Chromatic Order").
In 1969 Tabor & Winterton and later Israelachvili & Tabor developed Surface Forces Apparatuses (SFA's), initially for measuring the van der Waals forces between surfaces in air or vacuum. In 1976, Israelachvili & Adams designed the first apparatus (later known as SFA Mk I) for measuring forces between surfaces in liquids and controlled vapor atmospheres, allowing control and measurement of the surface separation to within 0.1 nm. The SFA Mk 1, and later versions (SFA Mks 2, 3 and 4), enabled the first direct and detailed measurements to be made of the fundamental forces between surfaces in vapors and liquids. Subsequent models, such as SFA Mk 2, SFA 3, SFA Mk 4, and other versions, were essentially incremental improvements on earlier models without introducing new qualitative features.
Current SFA technology--principles of SFA operation : The principles on which current SFA's operate are simple: one of the surfaces (usually the upper) is rigidly mounted at the end of a piezoelectric crystal tube while the lower surface, which faces the upper, is suspended at the end of a cantilever spring system (the force-measuring spring). The surfaces can be moved towards or away from each other using a two to four-stage system of mechanical and piezo controls of increasing accuracy, and an optical technique using FECO fringes is used to measure the separation between the surfaces to .+-.0.1 nm. The force between two surfaces is measured by moving the two surfaces towards or away from each other using one of the above controls, and simultaneously measuring the deflection of the force-measuring spring using the optical FECO technique. This gives the force at any particular surface separation.
The principles used in making direct force measurements are usually very simple, the main challenge has always been in the design of a mechanical device that would successfully apply these principles at the angstrom level.
FIG. 1 shows one of the more advanced basic SFA's, known as SFA 3, designed and used in the laboratory of the inventor at the University of California at Santa Barbara. This SFA will now be described in detail since one of its features--the movement/positioning of the lower surface in the z-direction--is also used in the present invention.
FIG. 1 shows a section through SFA 3. There are four distance controls: normal micrometer (M1), differential micrometer (M2), differential spring control (M3), and piezoelectric tube. The lower surface is mounted at the end of a variable-stiffness double-cantilever force-measuring spring (S) which is connected via the spring mount to the distance controls of the upper (control) chamber via a teflon bellows (B). The wheel and shaft are used for laterally moving the spring clamp and thereby changing the stiffness of the force-measuring spring. The lower chamber is bolted to the underside of the upper chamber from which it is completely sealed by the bellows, as well as being sealed from the outside with teflon O-rings. The main translation stage unit (T) has 4 double-cantilever springs machined out of a single Copper-Beryllium block. This part ensures that the two surfaces move vertically and perfectly linearly relative to each other with no displacement or rotation in any other direction. The main part of the translation stage is bolted to the inside of the control chamber. The second part is bolted to a single-cantilever spring which acts differentially to the helical spring when the differential micrometer M3 is rotated. All springs parts are machined from Cu--Be alloy before they are hardened by heat treated (tempered).
Forces are measured between the two mica or mica coated surfaces supported on two cylindrical silica disks. The upper disk is attached to the piezoelectric PZT-5A crystal tube which is mounted on a support that can be moved laterally and rotated before it is clamped tightly to the top of the apparatus. The apparatus has two separate parts, an upper (control) chamber, and a lower (bathing) chamber. The control chamber handles the four distance controls, the force-measuring spring adjustment, and the positioning and clamping of the two surfaces. Its workings are totally sealed from the lower chamber via the Teflon bellows B. The lower chamber acts as a simple bath that can be bolted underneath the upper chamber and then filled with liquid. It is thus completely sealed both from the outside environment as well as from the mechanical controls of the upper chamber. The lower and upper chambers are made of 316 stainless steel. The lower chamber can also be made of PTFE or some other inert material such as Kel-F. It is easy to clean and can be readily replaced by another bathing chamber. At no point during or between experiments does the control chamber have to be opened or dismantled; indeed, once the control chamber has been assembled it requires no further attention.
The four distance controls of the SFA 3 include three mechanical controls and one piezoelectric control. The three mechanical controls are based on a multiple spring translation assembly, located roughly at the center of the upper (control) chamber. The main part of this assembly is the vertical-motion translation stage, machined from a single block of Cu--Be alloy and consisting of four equal double-cantilever spring systems. This type of design ensures perfectly linear motion, with no possible movement in any other direction. The eight cantilever springs of this unit also ensure that there is no possibility of any wobble or rotation, or any other type of unwanted movement, friction or backlash as occurs with dove-tailed slides or screw-thread drives. To further ensure that there is also no "buckling" of the springs, the vertical force that induces displacement of the 8-spring unit is applied through an axis that passes through the center of the unit.
The full translation spring assembly includes a single cantilever spring that is bolted to the translation stage and whose center also passes through the center of the 8-cantilever spring unit. The assembled unit of these two parts (T) fits inside the control chamber via four bolts. At the bottom of this unit there is a threaded hole to which are attached the PTFE bellows (B) and the force-measuring spring mount. The spring mount protrudes into the lower chamber, and the bellows isolates the two chambers from each other.
Control of Surface Separation to 1 .ANG.: There are three mechanical distance controls (two coarse and one medium) and one piezoelectric (fine) control having the following resolution and workable range. Coarse control: normal micrometer M1 (500 nm over a range of 6 mm); Medium control: differential micrometer M2 (50 nm over a range of 0.1 mm); Fine control: differential spring operated by micrometer M3 (1 nm over a range of 5 mm); and Piezo control: piezoelectric tube (&lt;0.1 nm over a range of 1 .mu.m). The spindle of micrometer M1 and M2 presses against the 8-spring unit T via the short single-cantilever spring that is in series with that unit. When the spindle of micrometer M3 moves a distance D, it compresses a helical spring by this amount which in turn presses against the single-cantilever spring on unit T via the lever arm. The resulting vertical movement of unit T (and the spring mount) is therefore determined by the ratio of the stiffness of these two springs which, being 1,000:1, effectively gears down the motion of the lower surface by a factor of 1,000, viz. from D to D/1,000. The three micrometer controls can be operated manually or by variable-speed DC motors.
Measuring distances and surface profiles using the FECO technique: Surface separations and profiles can be measured to within about 1 .ANG. (0.1 nm) by monitoring the movement of multiple beam interference fringes. This technique is known as "FECO", for "Fringes of Equal Chromatic Order". These sharp interference fringes are produced when a beam of white light is made to pass through the two mica sheets (FIG. 1). The transparent mica sheets are of equal thickness (about 1-3 .mu.m thick) and are each coated with a highly reflecting layer of silver (thickness .about.550 .ANG.) before they are glued, silvered sides down, onto the cylindrically curved silica discs. The interference fringes are produced by multiple reflections of light between the two silvered layers, so that only certain wavelengths (those that interfere constructively) emerge from the other side of the sheets. The emerging beam is focussed onto the slit of a normal grating spectrometer which separates out the different colors (interference fringes). Depending on the shapes of the two surfaces these fringes appear as sharp lines or curves at the exit window of the spectrometer where they can be viewed by eye through a normal microscope eyepiece or recorded on film or a video camera.
From the positions and shapes of the FECO fringes one can determine not only the surface separation but also their shapes and the refractive index of the medium between them. Equations describing how one translates the measured wavelengths to surface separations and refractive indices have been described by lsraelachvili and others.
Measuring Forces: Given the facility for moving the surfaces towards or away from each other and independently measuring their separation, each with a sensitivity or resolution of 1 .ANG., the force measurements themselves now become straightforward. The force is measured by expanding or contracting the piezoelectric crystal by a known amount and then measuring optically how much the two surfaces have actually moved. Any difference in the two values when multiplied by the stiffness of the force-measuring spring gives the force difference between the initial and final positions. In this way, by starting at some large separation where there is no detectable force and working systematically towards smaller separations both repulsive and attractive forces can be measured with a sensitivity of about 10.sup.-6 g (10.sup.-8 N) and a full force-law can be obtained over any distance regime.
Advantages of the FECO technique over other distance-measuring techniques: There are two critical reasons for why the FECO technique is the only one capable of unambiguously measuring surface separations and inter-surface force-laws (force versus surface separation). The first is that the surface separation is measured at the point of closest approach of the two surfaces, i.e., the separation is measured exactly where one wants to know it. All other force-measuring techniques (whether capacitance, optical, electric or magnetic) do not measure the surface separation, that is, the distance between two surfaces, but the displacement of a single spring or balance arm at some point away from the interaction zone. From this displacement the separation is then inferred. In this way any elastic deformations of the surfaces occurring around the contact zone become mixed in with, and inseparable from, the measured force-displacement curves. With the FECO method, the distances between the two surfaces and any deformations of the materials are unambiguously distinguishable and measurable independently of the forces.
The second advantage of the FECO method is that one can measure the surface shape or profile, and in particular the local radius of curvature R of the two interacting surfaces. This is essential for comparing measured forces with theory or with other experiments.
FIG. 2 shows a sliding attachment (or "Friction device") that was developed in 1988 for use with the SFA Mk 2. This Friction Device allows for the two surfaces to be slid or moved laterally past each other (rather than normally as in conventional force measurements). Sliding motion is initiated via a motor-driven micrometer and the friction force is measured from the deflection of a double-cantilever spring system using resistance strain gauges. With this attachment, sliding speeds in the range 0.01 to 100 .mu.m/sec can be attained, and lateral (shear or friction) forces can be measured to an accuracy of about 10.sup.-3 N.
Limitations of the prior art: The systems and phenomena being called upon for study with SFA technology are rapidly growing in complexity: these now include complex polymeric and lubricant fluid systems, the interactions of biological cell surfaces, dynamic and time-dependent interactions over a large range of time-scales, molecular structure and relaxations at surfaces and in confined liquid films, etc. Such studies require greater versatility in the different types of forces that may be measured than has so far been possible. Thus, they require forces to be measured along different directions (axes), a higher force-measuring sensitivity, a greater dynamic range of measuring times and sliding speeds (from very slow to very fast), improved distance resolution, and higher mechanical stability (requiring the elimination of backlash and thermal drift). A whole generation of new phenomena could be studied if only some of these deficiencies or limitations could be overcome. Our primary aim here was the identification of these limitations, and the design and construction of a new device that overcame these limitations. These limitations will now be described in more detail.
(a) Inability to measure "vectorially" coupled interactions in 3-D. Recent research has indicated that normal forces (in the z-direction) and lateral forces (in the x- and y-directions) are generally "vectorially" coupled, for example, where sliding motion of one surface in the x-direction produces a force on the other surface in the y-direction. Existing SFA's cannot measure vectorially-coupled forces or interactions. More precisely, existing SFA's cannot simultaneously induce and/or measure forces along any desired (arbitrary) direction in three-dimensional space. PA1 (b) Inability to simultaneously measure normal and lateral forces accurately. Existing friction-measuring attachments replace, and therefore cannot be used together with, the sensitive piezoelectric tube supporting the upper surface. Thus, normal forces cannot be induced or measured accurately at the same time as shearing, sliding or friction forces are being measured. PA1 (c) Limited dynamic range of sliding speeds. Recent studies on friction and lubrication have indicated that important changes can occur at different sliding speeds, but that measurements have to be made over a very large range of speeds, extending over 10 decades or more, to fully appreciate these effects. Existing friction-measuring instruments or attachments cannot measure lateral forces over a range greater than about four orders of magnitude. PA1 (d) Inability to measure fast transient effects. Recent studies have shown that important short-lived "transient" effects occur during the interaction of two surfaces, and there is an increasing need for SFA's to be able to measure such rapidly changing, non-equilibrium forces. Existing force-measuring techniques are not geared to measuring rapid changes occurring over time-scales much shorter than 0.1 sec. PA1 (e) Inability to measure slowly changing and true equilibrium forces. The two surfaces are prone to thermal drifts which can greatly diminish the accuracy of the forces measured below the theoretically attainable limit of at least 10.sup.-8 N (10.sup.-6 g). These drifts can usually be minimized by thermostating the apparatus or the experimental room to 0.1.degree. C., but they preclude accurate measurements of forces that take a long time to reach equilibrium. Because of difficulties in controlling thermal drifts, existing SFA's cannot accurately measure forces that slowly change with time or forces that take a very long time to stabilize, for example, time-dependent adhesion forces, hysteretic forces, hydrodynamic interactions, etc. Thus, forces and interactions that require more than about 30 minutes to reach equilibrium cannot be reliably accessed by the prior art. PA1 (f) Limited interfacing capabilities, expandability and upgradability. Existing SFA's are not designed for interfacing with other commonly used experimental techniques, such as light scattering or fluorescence microscopy techniques, nor are they easily adaptable for incorporating new attachments that can expand their versatility and scope. PA1 (g) Limited accuracy in measuring normal forces. Currently the detection limit in measurements of normal forces is about 10.sup.-8 N. This is acceptable for most applications, but does not allow certain weak, but nevertheless important, forces to be measured. PA1 (h) Limited accuracy in measuring lateral forces. Currently the detection limit in measurements of lateral forces is about 10.sup.-3 N. This is fairly crude and precludes measurements of any but the strongest friction forces. PA1 (i) Limited accuracy in measuring lateral deflections. Currently the detection limit in measurements of lateral distances is about 500 .ANG.. This precludes measurements of any friction phenomena occurring or varying on the molecular scale. PA1 (a) Ability to measure vectorially coupled interactions in 3-D. The main object of this invention is to provide a multi-purpose force-measuring instrument capable of moving one surface (the driving surface) vectorially along any direction in three-dimensional space (i.e., along any desired x-y-z direction) while simultaneously measuring the static or dynamic force produced in the other (detector) surface and the direction of the induced force (which need not be the same as the direction of motion of the driving surface) and the surface separation and profile. PA1 (b) Ability to simultaneously measure normal and lateral forces accurately. Lateral, shear and friction forces can be measured without sacrificing the highly sensitive piezoelectric positioning-device supporting one of the surfaces. PA1 (c) Increased range of lateral sliding speeds. Sliding speeds can be varied from above 105 .mu.m/sec to below 10.sup.-5 .mu.m/sec--a range of 10 decades (orders of magnitude), which is 6 decades more than the prior art. PA1 (d) Increased time-resolution for measuring fast transient effects. By using friction-force measuring springs that are stiffer and shorter, and by drastically reducing the mass (weight) of the platform supporting the upper surface, the natural frequency of vibrations of the friction-force measuring system has been increased by more than an order of magnitude. Thus, transient events shorter than a few milliseconds can now be measured during sliding by recording the output signal from the strain gauges on the friction springs on or storage oscilloscope or other type of data acquisition system. This is a significant increase in response time resolution--by almost two decades--over the prior art. PA1 (e) Ability to measure slowly changing and true equilibrium forces. With the Balance Attachment, forces that may take hours, days or even weeks to equilibrate can now be measured. Additionally, very slow transient effects can also be reliably studied. In principle, the Balance Attachment can measure any equilibrium force or interaction, regardless of how long it takes to equilibrate and regardless of the thermal drifts present. The only limitation here being the patience of the operator. PA1 (f) Improved expandability, upgradability and interfacing capabilities. The new SFA is readily adaptable for incorporating a number of new attachments for further increasing the scope of force measurements. It is also readily adaptable for interfacing with other established laboratory measuring techniques such as X-ray synchrotron reflectivity and scattering experiments, and fluorescence and other optical microscopies. PA1 (g) Increased accuracy in measuring normal forces. With the balance attachment, the accuracy of measuring the normal forces between two surfaces moving towards or away from each other is increased by two orders of magnitude to 10.sup.-10 N. PA1 (h) Increased lateral force-measuring sensitivity. By using semi-conducting strain gauges and a friction-force measuring spring of variable stiffness, the accuracy of measuring the shear or friction forces between two surfaces moving laterally relative to each other has been increased by two orders of magnitude to 10.sup.-5 N. PA1 (i) Increased accuracy in measuring lateral deflections. The accuracy of measuring the lateral displacement of one surface sliding across another is increased by two orders of magnitude to about 5 .ANG., which now allows for friction phenomena and transient events to be studied at the molecular level. PA1 (j) Studies with opaque liquids. A facility has been introduced for adjusting the height of the light entrance window. This allows for a great reduction in the liquid volume that the light has to pass through (from cm to microns), thereby enabling opaque liquids to be used in SFA experiments. PA1 (k) Great interfacing capabilities with optical equipment. Enlarged objective port hole and &lt;5 mm optical working distance from surfaces to microscope objective allows for objectives with shorter focal lengths and higher magnifications (up to .times.20) to be used as well as certain types of specialized wide-bodied objectives, for example, fluorescence microscope objectives. PA1 (l) Unique features of SFA-FECO experiments: The SFA technique for measuring forces, when used together with the FECO optical technique for visualizing two surfaces, allows for the unambiguous measurements of the following parameters that cannot be independently measured by any other combination of techniques: (i) the static and dynamic forces, both normal and lateral, between the two surfaces, (ii) independent measurement of the separation between the two surfaces at their point of closest approach, (iii) quantitative visualization of the local surface geometry and how it changes, for example, due to force-induced elastic deformations, during sliding or other motions, arid (iv) the ability to visualize contamination and trapped particles between the surfaces from the deformed shapes of the FECO fringes in combination with refractive index measurements.